The document “A dual-mode Driver IC with Monolithic Negative Drive-Voltage Capability and Digital Current-Mode Controller for Depletion-Mode GaN HEMT” (Yue Wen and Al, IEEE Transaction on Power Electronic, 2016) describes, in particular, a switching device comprising a high voltage depletion mode transistor in series with a low voltage transistor in enhancement mode, each transistor being controlled separately.
The device comprises a first terminal, for receiving a switching signal, electrically connected, via a driver circuit, to the gate of the high voltage depletion mode transistor. The off-state of the high voltage depletion mode transistor is obtained by applying a negative voltage, below its threshold voltage, between the gate and the source of this transistor. As this negative voltage may not be available, for example, when turning on the switching device or when operating it in degraded mode, the device is equipped with a low voltage enhancement mode transistor, connected in series with the high voltage transistor.
The state-of-the-art switching device also includes a second terminal for receiving a control signal, the second terminal being electrically connected to the gate of the low voltage transistor. The control signal makes it possible to switch the low voltage transistor to the off-state and check the open state of the switching device, even in the absence of a negative voltage applied to the gate of the high voltage transistor.
By controlling separately, in “dual control” each of the high and low voltage transistors connected in series, it is therefore possible to define the open or closed state of the switching circuit.
It should be remembered that a switching device may have an “active” operating mode that corresponds to the “normal” operating mode wherein the open or closed state of the device is controlled by the switching signal. It may also have an “inactive” operating mode that corresponds to the “low power” mode (dormant device) or the device commissioning mode (starting the “active” operating mode).
In this “inactive” operating mode, the switching device is in the open state and the individual active elements of the device are not electrically powered. This is the case, for example, of the driver circuit connected to the gate of the high voltage transistor. If there is no power supply, this circuit has a high impedance output, the voltage that applies to the gate of the high voltage transistor is then floating and uncontrolled. A possible overvoltage on the drain of the high voltage transistor can then, by coupling effect between the drain and the gate, maintain or switch the high voltage transistor in/to an on-state despite the deactivation of the switching device. In this case, the entire switched voltage applies to the terminals of the low voltage transistor. This voltage can be significant, for example, greater than 600V, and exceed the avalanche voltage that this transistor can withstand, leading to damage to the device.
In addition, the change from one operating mode to another may result in transient voltages or transient current paths between the switching terminals of the device. These voltages or currents are likely to damage the high voltage transistor or the low voltage transistor if the change from one operating mode to another is not perfectly controlled.
More generally, it is desirable to have a driver circuit that can be configured to be perfectly adjusted to the other elements of the circuit and its environment in order to limit the consequences of transient phenomena that may occur when switching from one operating mode to another, or when operating the device.